Double metal cyanide complexes are well-known catalysts for epoxide polymerization. These active catalysts give polyether polyols that have low unsaturation compared with similar polyols made using basic (KOH) catalysis. The catalysts can be used to make many polymer products, including polyether, polyester, and polyetherester polyols. These polyols are useful in polyurethane coatings, elastomers, sealants, foams, and adhesives.
DMC catalysts are usually made by reacting aqueous solutions of metal salts and metal cyanide salts to form a precipitate of the DMC compound. A water-soluble, low-molecular-weight organic complexing agent, typically an ether or an alcohol, is included in the catalyst preparation. The organic complexing agent is needed for favorable catalyst activity. Preparation of typical DMC catalysts is described, for example, in U.S. Pat. Nos. 3,427,256, 3,289,505, and 5,158,922.
While water-soluble ethers (e.g., dimethoxyethane ("glyme") or diglyme) and alcohols (e.g., isopropyl alcohol or tert-butyl alcohol) are most commonly used as the organic complexing agent, many other general classes of compounds have been described. For example, U.S. Pat. No. 4,477,589 teaches (column 3, lines 20-22) that the organic complexing agent can be "an alcohol, aldehyde, ketone, ether, ester, amide, nitrile, or sulphide." Others list the same classes (see, e.g., U.S. Pat. No. 3,278,458 at column 6 and U.S. Pat. No. 3,941,849 at column 13). According to U.S. Pat. No. 3,278,458, the organic complexing agent preferably has "a substantially straight chain" or is "free of bulky groups." U.S. Pat. Nos. 5,158,922 (column 6) and 5,470,813 (column 5) add nitriles and ureas to the list of suitable complexing agents. Japanese Pat. Appl. Kokai No. H3-128930 (Morimoto et al.) teaches to use N,N-dialkylamides (e.g., N,N-dimethylacetamide) as the organic complexing agent to make catalysts with improved activity.
For decades, DMC catalysts having a relatively high degree of crystallinity were used for making epoxide polymers. The most popular catalyst contained an organic complexing agent (usually glyme), water, excess metal salt (typically zinc chloride), and the DMC compound. Activity for epoxide polymerization, which exceeded the activity available from the commerical standard (KOH), was thought to be adequate. Later, it was appreciated that more active catalysts would be valuable for successful commercialization of polyols from DMC catalysts.
Recent improvements in DMC catalyst technology have provided catalysts with exceptional activity for epoxide polymerization. For example, U.S. Pat. No. 5,470,813 describes substantially amorphous or non-crystalline catalysts that have much higher activities compared with earlier DMC catalysts. Other highly active DMC catalysts include, in addition to a low molecular weight organic complexing agent, a functionalized polymer such as a polyether (see U.S. Pat. Nos. 5,482,908 and 5,545,601) or other functional group-containing polymer (U.S. Pat. No. 5,714,428). Highly active DMC catalysts are generally substantially non-crystalline, as is evidenced by powder X-ray diffraction patterns that lack many sharp lines. The catalysts are active enough to allow their use at very low concentrations, often low enough to overcome any need to remove the catalyst from the polyol.
Even the best DMC catalysts known could be improved. High catalyst activity has sometimes come at a price, namely the unexpected formation of traces of polyether having extremely high (greater than 400,000 or &gt;400K) number average molecular weight (Mn). This high-molecular-weight component, even at part-per-million levels, can negatively impact the way polyether polyols made from the catalysts perform in urethane applications such as flexible or molded polyurethane foams. For example, polyols that contain too much high-molecular-weight component can process poorly, give tight foams, or cause foam settling or collapse. While various approaches have been proposed for dealing with the high-molecular-weight component (e.g., reformulation of the urethane, removal of the component from the rest of the polyol after formation), an ideal strategy would begin with the catalyst and minimize or eliminate formation of the component.
In sum, improved DMC catalysts are still needed. A preferred catalyst would have high activity similar to that of the substantially non-crystalline DMC catalysts now known (e.g., from U.S. Pat. Nos. 5,470,813 or 5,482,908). A preferred catalyst would still give polyol products with low viscosities and low unsaturation. Ideally, however, the catalyst would not produce significant amounts of high-molecular-weight polyol components, particularly those having number average molecular weights greater than about 400,000.